Barnes and Noble requires registration of your device to a www.bn.com account to be able to purchase and download NOOK content for the best possible user experience.
It is easy to register your NOOK. When you first turn on your NOOK, it will lead you through registration. In order to complete registration you'll need to have a BN.com account set up online with a valid default credit card and associated billing address.
If you defer registration when you first turn on your NOOK, you can register it later by going to Settings from your NOOK touchscreen, then tapping Device and then Register.
If you already have your NOOK registered to your BN.com account and want to make any changes to the email address or password for your account, please be sure to unregister your NOOK first. Then make the changes to the email address or the password of your account, and then re-register your NOOK using the current (new) email address and password for your account.